The present invention relates to visual display devices comprising an electrooptical material reacting to a mixed thermal and electrical effect and used for the display or production of images.
It is now well known to use a mixed thermal and electrical effect for controlling the orientation of molecules forming a liquid crystal, cf. particularly French Pat. No. 77 13 738, published under No. 2 389 955 and filed on May 5, 1977 by the Applicant company. This effect can also be used for controlling other electrooptical materials or for improving the performances of devices using them. This led the present Applicant company to file a French patent application under No. 81 23 437 on Dec. 15, 1981. This application proposes the reduction of the response time of an electrochromic display device and ensure a more definite threshold of the electrochromic effect by the combined use of a thermal effect and an electrical effect.
Heat can be supplied by a laser beam or the passage of an electrical current in the heating strips. The latter solution is being increasingly used, particularly in matrix display smectic liquid crystal devices. In a device of this type, it is advantageous for economy reasons and in order to reduce to the maximum the response time of the device, that the thermal energy supplied is not wasted in the plate facing the heating strips. However, when the addressing of an area of the screen corresponding to a heating strip is finished, it is also necessary that the heat accumulated in this area is removed by the plate supporting the corresponding heating strip. At present this is not carried out in display devices, where the plates supporting the electrodes forming a display matrix are either of the same nature, or are not chosen as a function of the aforementioned criteria.
In order to obviate these disadvantages, the use is proposed in these devices of plates chosen as a function of their thermal conductivity, the plate facing the heating strips having to dissipate a minimum quantity of the heat supplied to the electrooptical medium and the plate supporting said strips permitting a reasonable heat removal after the supply of heat has been stopped.